The present invention relates to displacement actuators and in particular to a thermally-activated displacement actuator useable in means for effecting temperature-compensation of, for example, the focal length of optical elements in an optical assembly.
Low-resolution, thermally-activated displacement actuators are commonly used on household appliances and automobiles as safety devices or as controllers of motion functions. A commonly-employed technique for effecting linear motion is to heat or cool a fluid confined within a cylinder having a moveable piston. The piston moves as the actuating fluid expands or contracts, as illustrated in U.S. Pat. No. 4,055,954. Control of actuator position by a temperature control based on position feedback is shown in U.S. Pat. No. 4,081,963. Another type of thermally-activated actuator employs phase changes in a force-generating medium to apply a force on a piston. The large volume difference between solid and liquid wax is disclosed in U.S. Pat. No. 5,025,627 wherein actuation is controlled via a thermoelectric cooler (TEC) based on position feedback. A volume difference in solid phases is disclosed in U.S. Pat. No. 4,553,393 wherein memory metal beams of binary weighted stiffness are resistively heated to cause a phase change, thereby creating a force proportional to the element axial stiffness. Still another type of thermally-activated actuator relies on expansion and contraction of solid members to achieve rotary motion, such as may be found in thermostats and automatic choke devices, wherein bimetallic springs produce rotary motion.
However, certain systems require precise displacement of a moveable mechanical or optical element to effect or maintain the accuracy or resolution of the system. For example, in a high-resolution optical imaging system, such as may be found in a laser output scanner, a stable, monochromatic collimated light beam must be provided by a beam source that typically includes a laser diode and a collimating lens. For adequate optical performance, the beam source must maintain a predetermined beam quality over a wide ambient temperature range. In conventional approaches, the laser diode and lens are mounted in a mechanical structure that attempts to maintain the beam focal length while the apparatus undergoes ambient temperature-induced structural changes. The thermal compensation of the beam focal length is typically effected either passively or actively.
Passive compensation systems rely on the differences in coefficients of thermal expansions of the various elements in the optical system such that there is a mechanical movement to minimize the net focus shift over a narrow ambient temperature range. One such conventional approach is to employ concentric tube systems, which, if constructed from common materials, are too large or bulky. For example, U.S. Pat. No. 4,730,335 discloses a series of interlocking tubes each carrying a single optical element of an optically-pumped solid-state laser. Such an apparatus can compensate only for a relatively narrow range of ambient temperature changes, and is too large to be suitable for many applications.
Conventional active compensation systems (wherein, for example, heating elements or thermoelectric coolers are used) have other disadvantages. For example, a thermoelectric cooler is employed in the apparatus disclosed in U.S. Pat. No. 4,604,753 to stabilize the output power and wavelength of a laser diode beam source; U.S. Pat. Nos. 4,656,635 and 4,993,801 disclose a beam source wherein a thermoelectric cooler is employed to control the operating temperature of the entire head. These apparatus are more complex and expensive to construct, and offer less accurate displacement, than is desired for certain applications, including the aforementioned laser output scanner application.